1. Field of the Invention
This invention relates generally to a telecommunication system, and, more particularly, to a wireless telecommunication system.
2. Description of the Related Art
Wireless telecommunication systems, such as cellular telephone systems, typically include one or more base stations that establish concurrent communication links with a plurality of mobile units. The range of a wireless communication link is limited by the transmission power of the base station and mobile units, the sensitivity of receivers in the base stations and mobile units, obstacles between the base station and mobile units, and the like. For example, if the wireless communication link is formed in free space, the power received at the mobile unit is approximately proportional to the power transmitted by the base station divided by the square of the distance from the base station to the mobile unit. For another example, the range of a base station located in a mountainous region may be substantially smaller than the range of the same base station deployed in a region that has very few physical obstacles. Obstacles such as buildings and mountains may also introduce a directional dependence into the relation between the transmitted and received power.
When the received power falls below a certain level, the mobile unit and the base station may be unable to form or maintain a wireless communication link. Thus, a relatively large number of base stations may be needed to provide complete coverage and uninterrupted service to users traveling through regions with large numbers of obstacles, such as mountainous regions and urban canyons. In addition to transmitting and receiving signals from the mobile units, a radio access network (RAN) associated with the base station may also perform other functions, such as coordinating hand-offs of moving mobile units with other base stations, maintaining lists of active mobile units associated with the base station, determining locations of the mobile units, and the like. These functions typically require additional hardware and/or software that increase the cost of deploying, maintaining, and operating the base station. Consequently, the cost of providing the large number of base stations that may be necessary to ensure complete coverage in regions having a large number of obstacles may be prohibitive. In particular, it may not be cost effective to provide a large number of base stations in a mountainous region having a low population density.
One conventional solution to this problem is to employ one or more repeaters associated with one or more of the base stations. The repeaters receive signals transmitted by the base station, amplify the power of the signal, and transmit the amplified base station signal to one or more mobile units that may be in a region beyond the range of the un-amplified base station signal. The repeaters may also receive signals transmitted by the mobile unit, amplify the power of the mobile unit signal, and transmit the amplified mobile unit signal to the base station. However, repeaters typically do not perform many of the other functions of a base station. Consequently, repeaters may be used to extend coverage area of the base station at a much lower cost than would be incurred by installing additional base stations. The additional coverage area provided by the repeater is often referred to as a repeater cell.
Repeaters also introduce time delays into the signals. For example, due to the close proximity of the receive antenna and the transmit antenna in a repeater, repeaters often have to remove and/or cancel the repeater transmit signal from the received signal to avoid or reduce feedback interference. Canceling the transmit signal is done using digital signal processing, which introduces a time delay between the received signals and the signals transmitted by the repeater.
Conventional radio access networks associated with conventional base stations are typically unable to determine whether a received signal passed through a repeater. The digital signal processing performed by the repeater is transparent to the wireless communication system. Thus, the radio access network is not able to distinguish between propagation-related time delays and time delays introduced by signal processing. And even if the time delay associated with the digital signal processing by the repeater is detected by the radio access network, the radio access network will interpret the delayed signal as multipath reception with the strongest path not being the direct path from the base station to the mobile unit. The conventional radio access network is also unable to detect the signal amplification provided by the repeater.
Consequently, the time delay and/or the signal amplification provided by the repeater may reduce the accuracy of location services provided by the conventional radio access network. For example, the radio access network may estimate the distance to the mobile unit using the round-trip time delay of a signal. However, conventional radio access networks overestimate the distance to the mobile unit when a repeater has processed the signal because the conventional radio access network is not able to compensate for the additional time delay introduced by the repeater. For another example, the radio access network may attempt to measure the distance to the mobile unit by comparing the transmitted power with the received power and applying an appropriate channel model for the wireless communication link. However, conventional radio access networks applying the channel modeling technique will underestimate the distance to the mobile unit because the conventional radio access network is not able to compensate for the power amplification introduced by the repeater.
The present invention is directed to addressing the effects of one or more of the problems set forth above.